It is known that certain substances denominated catalysts, when present in trace amounts on the surface of a suitable refractory material, can bring about the flameless oxidative combustion of fuels. The relevant group of catalysts includes, but is not limited to, platinum, palladium, silver, zirconium, rhodium, vanadium, iron, nickel, lanthanides, actinides, oxides of the preceding, and carbon black. The combustion process takes place on the surface of the catalyst or catalytic substrate, and appears in part to involve a reaction mechanism whereby the catalyst induces the formation of monoatomic, or "free-radical," oxygen on its surface at low temperatures, e.g., -20.degree.-300.degree. C. In this form, the oxygen readily combusts fuels that come in contact with the surface. Only trace amounts of the catalyst, e.g., 0.05% to 0.2% of platinum by weight on ceramic beads, are required to produce this effect. Another complimentary catalytic process involves the action of materials having high "surface energy," which tend to be hard, high melting point substances as glass, ceramic, polished chromium, etc. The presence of this "surface energy," in the form of uncompensated electric fields associated with the valence electrons of the surface atoms, allows such a surface to capture and hold molecules, such as fuel or water, to await oxygenation or vaporization. Such molecular capture may also have the effect of lowering the activation energy required to initiate the combustion reaction. The combustion process, thus enhanced by the aforementioned catalytic effects, is virtually 100% complete and leaves no residue, even in the case of fuels that normally require much higher combustion temperatures. Because it is a surface phenomenon, the burn may also be induced to occur in a fixed place, area, or volume, and be given a specific shape, as may be advantageous for the desired result.
The prior art teaches a number of applications for this basic combustion technique. It is used in furnaces and radiant heaters for the combustion of gaseous fuels in industrial and residential settings, in off-gas burners and recombiners to ignite and combust undesired process wastes, in exhaust purifiers to completely combust carbon monoxide and other incompletely burned combustion products that would otherwise be pollutants, and in fluid heaters, boilers and vapor generators to apply the heat of the controlled surface burn to a pipe, tube, or other vessel containing a fluid to be heated or vaporized. (See, e.g., U.S. Pat. No. 3,908,602.)
Another category of catalytic combustion applications may be characterized as ignition devices. Such applications have included the use of catalyst to initiate burning of fuels at extremely low temperatures, to generate atmospheres consisting of CO.sub.2, steam, and hot air, or to ignite an explosive mixture of air and natural gas at the bottom of a well to eject water seepage during drilling operations. (See U.S. Pat. No. 3,070,178.)
In regard to inventions that relate to the vaporization of water with the heat of catalytic combustion, it has been generally assumed that the fluid to be vaporized must remain physically removed from the surface on which the combustion is taking place. Typically, this results in process embodiments wherein the water or fluid to be vaporized is contained in boiler tubes or circulation pipes, which are then routed through the site of combustion. Alternatively, the tubes or vessels may be catalytically doped so that their exterior surfaces become sites of combustion. The heat transfer process through the wall of such a tube is generally much better than would have been obtained by placing the untreated boiler tube in an ordinary flame. However, our research has indicated that it is not necessary to physically separate the combustion process from the vaporization process, which can in fact occur in the same surface, even with a large excess of water. If distilled water is used its vaporization does not leave residues that poison or foul the catalytic element. The use of vanadium oxide, in addition to the principal catalyst, facilitates combustion of sulfur impurities in fuels, to prevent formation of those residues. Other fuel-contaminant-specific catalysts may also be employed as needed. As will be more fully set out below, this new conceptual basis for steam generation allows for the creation of new processes and apparatus incorporating a number of substantial machine design and thermal efficiency advantages over previous steam generation processes.